Take the press kit NASA prepared for the GRAIL mission. GRAIL consists of two nearly identical spacecraft that are on their way to the moon. Once there, they will make a precise map of the moon's gravitational field. Such a map will help scientists refine their theories about how the moon formed and what the interior is made of.

So I'm reading through the GRAIL press kit, learning about the size of the spacecraft (washing machine-sized), how long it will take to make the map (approximately three months), and where the mission control room is (at a Lockheed Martin facility just outside of Denver).

I also learn that GRAIL can launch any day between Sept. 9 and Oct. 19. And then I get to this sentence:

"On each day, there are two separate, instantaneous launch opportunities separated in time by approximately 39 minutes."

That's weird. Now, I wasn't surprised that this mission had an extremely precise launch opportunity — "instantaneous" means the launch window is essentially a second long. I know you have to wait until the moon's orbit around the Earth, and Earth's orbit around the sun, and the Earth's rotation around its axis are all in exactly the right position before you can launch your rocket.

But what's up with two instantaneous launch opportunities. Why 39 minutes apart?

I called the NASA press office. They were stumped and promised they'd find someone who could give me the answer. In the meantime, I called several universities with large aerospace programs, and asked if anyone there had the answer.

Nope. Nada. Nothing.

So now this is beginning to bug me. I'm also beginning to worry. If an aerospace engineer can't figure this out, how am I ever going to understand it?

While I'm pondering this, GRAIL deputy project manager Tom Hoffman called. I asked him about the two instantaneous launch opportunities.

"It's a pretty simple answer not driven by the mission at all," Hoffman told me. "It's driven by the launch vehicle."

The launch vehicle? Not orbital mechanics or a complex trajectory? The problem, Hoffman said, is some slow electronics.

There is actually a launch window of 40 minutes or so each day. Modern computers in modern rockets can calculate the precise direction the rocket needs to launch on a second-by-second basis. But GRAIL was launching on a Delta II rocket, and Hoffman told me the Delta II rocket is a rather old design. It's been around for more than 20 years, "and so the rocket just can't, it can't do that internally, in its own computer system. It basically has to be fed all the information," he said.

Hoffman said it takes about 39 minutes to load in the new information, and then check to make sure that it was loaded correctly.

Not the answer I was expecting, but there's something oddly pleasing about a brand-new space mission launching with some rather old electronics.

Whatever works ... as the saying goes.

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